@Clubjack and others,
I noticed this problem too.
For instance my birthday lasted 2 days ( only got presents once though) and my wedding-day is one day forward (which will upset my wife).
The entries were made using Google Calendar (so not a calendar app that synchronizes with Google).
I notice this since 2020, but that does not mean it did not happen before, could have overlooked it.
When I ran a test with a dummy entry, it worked fine.
When I toggle a „faulty“ entry from repeating to not repeating and then back to repeating, the entry is shown correct.
I collected some data and will post it when I sorted it out a bit.